Effect of water addition and nitrogen fertilization on the fluxes of CH4, CO2, NOx, and N2O following five years of elevated CO2 in the Colorado Shortgrass Steppe A. R. Mosier1, E. Pendall2, and J. A. Morgan11USDA/ARS, Fort Collins, CO, USA2Department of Botany, University of Wyoming, Laramie, WY, USA

Abstract. An open-top-chamber (OTC)
CO2 enrichment (~720 mmol
mol-1) study was conducted in the Colorado shortgrass steppe from April 1997 through October 2001.
Aboveground plant biomass increased under elevated CO2 and soil moisture content was
typically higher than under ambient CO2 conditions. Fluxes of
CH4, CO2,
NOx and N2O,
measured weekly year round were not significantly altered by CO2 enrichment over the 55
month period of observation. During early summer of 2002, following the removal of the
open-top-chambers from the CO2 enrichment sites in October 2001, we conducted a short
term study to determine if soil microbial processes were altered in soils that had been exposed
to double ambient CO2 concentrations during the growing season for the past five years.
Microplots were established within each experimental site and 10 mm of water or
10 mm of water containing the equivalent of 10 g m-2 of ammonium nitrate-N was applied to the soil
surface. Fluxes of CO2,
CH4, NOx and N2O fluxes within control
(unchambered), ambient CO2 and elevated
CO2
OTC soils were measured at one to three day intervals for the next
month. With water addition alone, CO2 and NO emission did not differ between ambient and
elevated CO2 soils, while
CH4 uptake rates were higher and N2O fluxes lower in elevated
CO2
soils. Adding water and mineral N resulted in increased CO2 emissions, increased
CH4 uptake and decreased NO emissions in elevated CO2 soils. The N addition study confirmed
previous observations that soil respiration is enhanced under elevated CO2 and N
immobilization is increased, thereby decreasing NO emission.

Citation: Mosier, A. R., Pendall, E., and Morgan, J. A.: Effect of water addition and nitrogen fertilization on the fluxes of CH4, CO2, NOx, and N2O following five years of elevated CO2 in the Colorado Shortgrass Steppe, Atmos. Chem. Phys., 3, 1703-1708, doi:10.5194/acp-3-1703-2003, 2003.